Process of carburizing iron or steel



Patented Aug. 8, 1-933 1,921,367 7 PROCESS OF CARBURIZING IRON R STEEL Edward G. Mahin, South Bend, Ind.

No Drawing. Application January 6, 1930 Serial No. 418,981

12 Claims. (Cl. 148-19) This invention relates to carburization or case hardening of iron and steel.

Before proceeding with the presentation of the present invention, it is desirable to review the fundamental factors involved in the cementation of iron or steel by carburizing materials.

Considering the character of the case in general, this varies in accordance with the requirements of the case carburized article. Since the primary object of all surface carburization is to produce a hard wearing surface, without destroying the ductility of the body, it may be assumed that the usual aim is the production of a case of the necessary depth, and high carbonexterior layers, without excessive brittleness of the surface of the heat treated piece.

Relative to the depth of case, with steel of a given composition, the principal factors affecting the rate of carbon absorption at the surface are temperature and nature of carburizing compound. In general, the rate of absorption increases with rise of temperature. The rate of inward migration of the absorbed carbon depends upon the temperature and concentration gradient, although the latter factor appears to be of less importance than is that of temperature. The total depth of case increases with time of heating, rise in temperature and concentration gradient but, here again, time and temperature appear to be the more important factors.

Considering the properties of the case itself, it may be stated that having established, as a result of commercial practice, the necessary depth of case for a given service, there remains the question as to the carbon distribution within the case. The outer surface should have as high carbon content as possible, without imparting undue brittleness after thermal treat- 40 ment, in order to confer upon the surface the maximum practicable resistance to wear or abra-'- sion. Where the piece is to be subjected to no great stresses, higher carbon content is permissible than otherwise, and hypereutectoid outer zones may carry as much as 1.8% of carbon. For a great many other uses, the outer case should not carry more than about 0.9% of carbon, so that a microscopic examination of a cross section would indicate an eutectoid outer zone passing into the usual hypoeutectoid zone with carbon decreasing toward the inner line of the case.

The prcccnt invention contemplates in the carburization of iron and steel articles. a control of the absorption of carbon at the outer thin skin of the case. There is a reduced rate of absorp- Cal tion at the surface which does not materially ditime necessary for production of a given case depth, and at the same time eliminating the brittlenessincident to a hypereutectoid outer case. It may be stated that it is well known that a higher temperature for carburization is desirable in order to shorten the time, but it could not, prior to the present invention, be used since it caused the building up of an excessive carbon containing entity at the outer skin layer of the case. However, with a control of the absorption of carbon at the outer skin layer, this disadvantage is eliminated.

The present invention therefore, in its broadest aspect, comprises the use of higher temperatures than usual in the carburization of iron and steel articles by controlling the absorption of carbon at the outer skin layer of the case, thereby eliminating brittleness incident to the accumulation of carbon at the outer skin layer and reducing the time necessary for carburization, while at the same time the reduced rate of absorption at the surface does not materially diminish the migration rate of the carbon after it has passed through the very thin surface case layer, this rate having been accelerated by the use of the higher temperature.

More specifically, the present invention contemplates the production of carburized iron and steel articles having a given or predetermined case depth, and characterized by the absence of brittleness in the outer skin layer, this being accomplished by effecting the carburization in the presence of a control agent and at a higher temperature and for a shorter period of time than is usually employed, the standard temperature hitherto used being approximately 890 C., and the time about 13 hours. According to the process herein set forth, the temperature may be considerably higher and may be approximately, in some cases, 950 C., the time being reduced to approximately four or. five hours. The new temperature and time of carburation obviously will vary somewhat with the character of the material treated, and the control agent utilized, andare to be taken as illustrative and not by way of limitatio. 1.

According to the present invention, thereisal so provided a suitable carburizing medium ing a control agent for effecting the control of the absorption of carbon at the outer skin layer of the case, and the same will be hereinafter more specifically described.

It has been discovered that a number of agents will retard the absorption of the carbon at the outer skin layer of the case. Ferrosilicon is a typ ical agent, and the present invention will be illustrated in connection with this material. While under some conditions it may bepossible to carry out the present invention by having the ferrosilicon or equivalent reagent in direct contact with the iron or steel articles to be carburized, experiments indicate that the preferred mode of proceeding is to keep the ferrosilicon particles out of contact with the article to be carburized, and in this manner prevent direct diffusion by contact. This may be effected by coating the ferrosilicon with a suitable material, and mixing the coated ferrosilicon with the carburizer, or the former may be coated and inserted in the carburizing box after surrounding the article to be carburized with a carburizing agent. Again, ferrosilicon may be mixed with a protective coating material and applied directly to the iron or steel, the usual carburizer also being present.

Instead of using a solid carburizer, gaseous carburization may be employed and CO gas passed over ferrosilicon held out of direct contact in any suitable manner with the article to be carburized.

In practicing the invention, the ferrosilicon is preferably incorporated in the carburizing material although, as indicated, the article to be carburized may be surrounded with a carburizing agent and the ferrosilicon merely be present in the carburizing vessel. Preferably, when employing the latter procedure, the ferrosilicon has a protective covering or coating, although this is not absolutely necessary. The ferrosilicon may also be mixed with a coating material, and the mixture applied in the form of a paste directly to the iron or steel article, a carburizing agent of course being present.

Any suitable carburizing agent may be used. Preferably, it is desired to use the shell coated carburizer set forth in U. S. Patent No. 1,550,952, issued to Peter C. Reilly. Proceeding in accordance therewith, the ferrosilicon is incorporated in the carburizing particle, which may contain any of the well known activators such as barium or calcium carbonate, and then the resulting particle carrying the incorporated ferrosilicon is coated with a non-carburizing material such as pitch, which is then carburized to produce a porous shell surrounding the particle. This is the preferred mode of preventing actual contact between the ferrosilicon and the article to be carburized. However, in general, any suitable carburizing agent may be used.

The amount of ferrosilicon present in the modified carburizing agent may vary within rather wide limits. Satisfactory results have been obtained using 5 to 10% of ferrosilicon and 95 to 90% of carburizer. However, in a number of runs, the ferrosilicon content has been as high as 20%, and it may be considerably higher.

Ferrosilicon may vary considerably in its silicon content. In commerce, the ferrosilicon is designated by its silicon content. For example, ferrosilicon indicates a silicon content of 50%. In practicing the present invention, satisfactory results have been obtained using 15%, 50% and ferrosilicon. In general, while the above constitutes the preferred ferrosilicon, it

long, the steel being packed with about 100 to is not desired to .be limited to the use of any particular ferrosilicon.

As an alternative procedure, in practicing the present invention, the ferrosilicon may be mixed with a coating agent which will stand the temperature of carburization without decomposition, and applied directly to the article to be carburized. Any suitable coating agent may be used such as coal tar pitch, or an alkali silicate. When using .an alkali silicate, the preferred procedure is as follows: A paste is made from 100 cubic centimeters of water-glass and 100 grams of ground ferrosilicon. This paste is applied to the article to be carburized, and the coated article packed in a suitable carburizer, as hereinbefore set forth.

Utilizing either of the working agents, the carburization is carried out at about 950 C., which is considerably higher than the temperature ordinarily used. This higher temperature accelerates the carbon migration and-shortens the time necessary for the production of a given case depth, while at the same/ time eliminating brittleness incident to the accumulation of carbon, at the outer case skin layer. This would occur if the higher temperature had been used without the presence of an agent such as ferrosilicon or its equivalent, acting to control the absorption of carbon at the outer skin layer. The employment of the higher temperature. which in the example given is approximately 60 C. or 180 F. higher than the usual temperature, enabled the time of the carburizing operation to be reduced from approximately thirteen hours to five hours. This new departure represents a decided saving in time, and is a tremendous advance in the carburizing art. The basic point to be kept in mind is that the ferrosilicon used as illustrative in the above description of the control agent, effects a reduction of the absorption of the carbon at the outer thin skin layer of the case, thereby preventing an accumulation of carbon at the outer case surface, and after the carbon has passed through the surface layer, the migration rate of the carbon is accelerated by the higher temperature employed.

In practicing the invention, while it has been found that any size of ferrosilicon is efiective, it has also been discovered that the activity of the ferrosilicon, or its equivalent, is greater, the smaller the size of the particle. The activity of 125 the ferrosilicon, or its equivalent, appears to be approximately aninverse function of the particle size. This will be more clearly hereinafter brought out by a presentation of the results obtained from actual runs. I

It is desired to set forth the typical results obtained in carburizing operations, in which ferrosilicon is used in accordance with the present invention as previously set forth.

In the runs. the results of which are hereinafter tabulated as tables 1, 2 and 3, the ferrosilicon employed was, in most instances, a 50% ferrosilicon. The runs were made on pieces of steel of an inch in diameter, and 2 inches l50'grams of carburizing material containing 20% of ferrosilicon. The duration of the runs were four hours, and the temperature employed was The results may best be expl'essed by stating in millimeters the total depth of case, the depth of the hypereutectoil case, the eutcctoid case, and the hypoeutectoid case. Accordingly, in the following tables, the abbreviations Hyper. case, Eut.

case, and Hypo. case, designate respectively the depth in millimeters of the hypereutectoid ,case,

the eutectoid case, and the hypoeutectoid case.

The following table, No. 1, sets forth a typical run with a standard carburizer, which does not have incorporated therein a control agent, such as ferrosilicon:

Table No. 1 7

Total Hyper. Eut. Hypo. Run depth C356 C388 C858 02 case It is to be noted that the depth in millimeters of the hypereutectoid case is relatively high, indicating an accumulation of carbon at the outer skin surface of the steel piece carburized.

The following table No. 2 sets forth the results of runs using the same 20% ferrosilicon carburizing material in successive heats to show the permanent modifying activity of the ferrosilicon.

Table No. 2

Mesh of Total Run ferroig depth silicon of case It ist o be noted that the depth of the hypereutectoid case is extremely low and, in some cases,

' practically nil.

Table No. 3

Mesh Total Percent of ferrosilicon in size of Hyper. Eut. Hypo. de th the carburizer ferrocase case case or g silicon hours. The carburizer contained 20% of 50% ferrosilicon.

Table No. 4

Total Hyper. Eut. Hypo. Mesh size of ferrosillcon case case case case depth In the above table, the expression 100-200 mesh indicates that the ferrosilicon passed through a 100 mesh sieve and was retained on a 200 mesh sieve. The same interpretation is to be given to the remainder of the mesh sizes specified in this table for the ferrosilicon.

In the present disclosure, there has been set forth the modified carburizing material used, containing an agent for controlling theabsorption of the carbon at the outer thin skin of the case; the preparation of the same; the method of using the same; and the tabulated results of several runs. The requirements of the statutes have thereby been complied with. While it is not desired to be bound by any theory of what actually occurs, it is thought that there is a gaseous transfer of silicon from the ferrosiliconto the steel. It has been found that there is a fairly constant, though small gain in the silicon content of the surface layers of steel carburized in accordance with the present invention, using either solid or gaseous that is, carbon monoxide) carburization. The analyses which have been made indicate that the silicon has penetrated only the extreme outer layers of the iron or steel. However, even such thin layers of high-silicon iron have so retarded the absorption of carbon as to practically eliminate, or substantially reduce, the depth of the hypereutectoid zone, while permitting undisturbed inward migration of the carbon after absorption. As previously stated, in this disclosure ferrosilicon has been selected as an illustrative control agent for controlling the absorption of the carbon at the outer thin skin of the case, while permitting the undisturbed inward migration of the carbon after absorption. The present invention is not limited to the use of ferrosilicon. Other control agents which may be used are magnesium silicide, the silicon hydrides, and elementary silicon. Boron will function similarly to silicon, and the boron may be in the form of ferro-boron or other boron compounds. Particularly satisfactory results have been obtained when using as a control agent an alloy analyzing 47.86% silicon, 37.06% zirconium, aluminum 1.51%, carbon 0.30%, iron (by difference) 13.27%.-

It is to be understood that the present invention is applicable to ordinary carbon steels and special steels commonly used for carburizing stock such as chromium steel, nickel steel, and chrome nickel steel. In general, the present invention is applicable to the production of a hardened case on iron and steel material of articles of every description, and in the claims the expression iron or steel articles covers raw materials or articles made from iron, steel, iron alloys and steel alloys.

What is claimed is: a

1. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon diffusion without building up high carbon concentration in the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent, and an absorption-retarding agent functioning to reduce the absorption of carbon at the outer thin case of the article, said carburization being effected at a temperature substantially above the temperature now employed, the latter being approximately 890 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

2. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high' carburizing temperature and thereby accelerate carbon diffusion without building up high carbon concentration in the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent and an absorptionretarding agent functioning to reduce the absorption of carbon at the outer thin case of the article, the carburizing agent being present in a predominating proportion, said carburization being effected at a temperature substantially above the normal temperature now employed, the latter being approximately 890 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

3. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the articles to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon diffusion without building up high carbon concentration inthe extreme outer layer of the carbon case by carburizing in the presence of a mixture of a carburizing agent and an absorption-retarding agent functioning to reduce the absorption of carbon at the outer thin case of the article, the carburizing agent constituting at least 80% of the mixture, said carburization being effected at a temperature substantially above the normal temperature now employed, the latter being approximately 890 C., and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

4. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon diffusion without building up high carbon concentrationin the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent, and a siliconcontaining absorption-retarding agent function-- ing to reduce the absorption of carbon at the outer thin case of the article, said carburization being effected at a temperature substantially above the temperature now employed, the latter being approximately 890 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

5. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperature and therebyaccelerate carbon diffusion without 'building up high carbon concentration in the extreme outer layer of. the outer case of the articles by carburizing in the presence of a carburizing agent, and a ferro-silicon absorption-retarding agent functioning to reduce the absorption of carbon at the outer thin case-of the article, said carburization being effected at a temperature substantially above the temperature now employed, the latter being approximately 390 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

6. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon diffusion without building up high carbon concentration in the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent, and a finely divided silicon-containing absorption-retarding agent functioning to reduce the absorption of carbon at the outer thin case of the article, said carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon diffusion Without building up high carbon concentration in the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent, and an absorption-retarding agent carrying a coating which will withstand the temperature of carburization and functioning to reduce the absorption of carbon at the outer thin case of the article, said carburization being carried out at a temperature substantially above the normal temperature now employed, the latter being approximately 890 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours. 7

8. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the'article to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon diffusion without building up high carbon concentration in the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent, and a siliconcontaining absorption-retarding agent carrying a coating which will withstand the temperature of carburization and functioning to reduce the absorption of carbon at the outer thin case of the article, said carburization being carried out at a temperature substantially above the normal temperature now employed, the latter being approximately 890 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

9. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon diffusion without building up high carbon concentration in the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent, and a ferro-silicon absorption-retarding agent carrying a coating which will withstand the temperature of car'- burization and functioning to reduce the absorption of carbon at the outer thin case of the article, said carburization being carried out at a temperature substantially above the normal temperature now employed, the latter being approximately 890 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

10. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperat re and thereby accelerate carbon difiusion Wit out building up high carbon concentration in the extreme outer layer of the outer case of the articles by coating the article with a mixture containing an absorption-retarding agent functioning to reduce the absorption of carbon at the outer thin case of the article, and an agent which will withstand the temperature of carburization, and carburizing in the presence of a carburizing agent and said absorption-retarding agent, the carburization being effected at a temperature substantially above the normal temperature now employed, the latter being approximately 890 C. and for a shorter period of time than is the current practice, the latter being approximately 13 hours.

11. The process of carburizing iron and steel articles comprising retarding the absorption of carbon at the outer thin case of the article to permit the employment of a relatively high carburizing temperature and thereby accelerate carbon difiusion Without building up high carbon concentration in the extreme outer layer of the outer case of the articles by carburizing in the presence of a carburizing agent, and an absorption-retarding agent, out of contact with the article and functioning to reduce the absorption CERTIFICATE-0F CORRECTION. I

Patent No. 1,921,367.

August 8, 1933.

EDWARD G. MAHIN.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction asfollowsz Page 2,

line 105, for

"180" read "108"; page 4, line 64, claim 5, for "390" read "890"; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 26th day of September, A, D. 1933.

I (Seal) F. M. Hopkins Acting Commissioner of Patents. 

